1. Field of the Invention
This invention relates to methods for decreasing oxidative melt loss of aluminum and magnesium melts and, more particularly, this invention relates to methods for providing inert physical and physico-chemical barriers to melt oxidation in aluminum and magnesium processing.
2. Background of the Invention
Almost all aluminum is melted and cast during its life cycle. During the melting process, significant aluminum loss occurs due to oxidation at the surface. Melting furnaces are large objects (3mxc3x975mxc3x975m high). Typically, they have side doors that allow a front-end loader to drop scrap aluminum into the furnace. Alternatively, the scrap is loaded through a removable roof into the hot furnace.
During loading, the furnace often contains some molten aluminum (called the heel). This facilitates melting of the scrap aluminum, which takes several hours. However, when the chemistry of the bath needs to be modified, a heel is not utilized and loading occurs in ambient temperature.
Once the scrap aluminum is melted, any aluminum oxide (called xe2x80x9cdrossxe2x80x9d) formed on the molten aluminum surface is physically scraped off or otherwise removed. The molten aluminum is then transferred to a holding furnace and then passed through several filters to remove entrained alumina. Finally, the filtered aluminum is cast.
When primary ingot is melted to form products, the surface of the melt starts to oxidize. Typically the losses from this operation are about 5 percent. The kinetics of this oxidation behavior of aluminum alloys are well-known and the initial growth is characterized by a period of slow growth that is two-dimensional (2-D), or amorphous, followed by a period of rapid growth characterized as three-dimensional (3-D), or crystalline.
Material lost during aluminum and magnesium processing typically takes three forms in the furnace:
1.) Dross, a mixture of aluminum oxide compounds and aluminum metal typically skimmed from the surface of the melt;
2.) Inclusions entrained in the molten metal and removed by filtration; and
3.) Oxide sludge found at the bottom of the melt.
The amount of aluminum that is lost as dross depends on many factors, such as furnace operation (e. g., temperature, atmosphere, stirring, etc.) and the specific surface area, alloy composition, and cleanliness of the input feed. The annual net amount of aluminum lost as dross totals in the vicinity of more than 330,000,000 pounds. Because aluminum requires approximately 13 kWh per kilogram to produce from bauxite, this metal loss represents an equivalent energy loss of nearly 2,000 MWh (12.5 trillion Btu/yr).
Present methods for decreasing aluminum and magnesium melt losses have several drawbacks. For example, beryllium or boron can be added to the aluminum alloy melt. As little as 0.01 wt. percent beryllium can inhibit the oxidation of a 3.5 wt. percent magnesium alloy of aluminum for at least 46 hours at 800xc2x0 C. Without the beryllium, xe2x80x9cbreakawayxe2x80x9d oxidation can occur. However, the formation of an oxide IB film in the form of crystalline magnesium aluminate can overcome the protective action of even 0.01 weight percent beryllium in this same alloy at 800xc2x0 C. C. N. a-Cochran et al.,xe2x80x9cOxidation of Aluminum-Magnesium melts in Air, Oxygen, Flue Gas, and Carbon Dioxide,xe2x80x9d Metallurgical Transactions B, Vol 8B, June 1977, pp.323-332.
The use of boron in melt scenarios also stymies aluminum and magnesium oxidation, as discussed in D. L. Belitskus, xe2x80x9cEffect of H3BO3, BCI3, and BF3 Pretreatments on Oxidation of Molten Al-Mg Alloys in Air,xe2x80x9d Oxidation of Metals, Vol. 8, No. 5, 1974, passim. Unlike beryllium, boron does not preferentially oxidize from the alloy to concentrate in the surface film. Boron can be used in the form of H3BO3 (dusting on sample surface), dipping of sample in aqueous H3BO3, or by passing BCI3 over the sample""s surface.
Notwithstanding the foregoing, both beryllium and boron have toxicity problems which require special training and processing considerations.
A need exists in the art for a method to significantly reduce oxidative losses from dross formation in remelts of aluminum and magnesium ingots. Such a method would employ a physical or chemical-physical barrier to greatly diminish the annual metal losses which presently occur, along with the concomitant energy losses. The method would also employ micro-alloying elements which have little or no toxicity as opposed to beryllium and boron.
An object of the present invention is to provide a method of decreasing oxidative melt loss during secondary aluminum and magnesium production processes that overcomes many disadvantages of the prior art.
Another object of the present invention is to provide a method for forming protective films on aluminum and magnesium melts during melt processes. A feature of this invention is the coherent growth, in situ, of a thin, defect-free film that is a barrier to further oxygen transport. An advantage of the invention is that the resulting thin dross films exhibit significantly reduced oxygen diffusion rates.
Still another object of the present invention is to provide a method for creating a physical barrier between the surface of molten metal and the environment during metal processing. A feature of the invention is floating discrete entities (such as hollow spheres) of inert, low density materials on the melt surface. An advantage of this invention is that the spheres would limit oxide growth.
Another object of the present invention is to provide a method for extending the nucleation time for three-dimensional oxide growth beyond the typical industrial melt and pour cycle time in metal production processes. A feature of the invention is that the entities floating in the melt reduce melt surface exposure to the furnace atmosphere. An advantage of the invention is that the low-growth-rate incubation period of dross growth is extended beyond the melt cycling time without significantly affecting loading and melting operations. An additional advantage of this invention is a reduction in energy loss which would otherwise occur due to the oxidation.
Still another object of the present invention is to provide methods for decreasing oxidative melt loss during secondary aluminum and magnesium processes. A feature of the invention is to utilize minor alloying constituents, including, but not limited to, lithium, magnesium (for aluminum processing), calcium, and sulfur in combination with solid-phase and inert floatable materials to facilitate the selective formation of compounds on the surface of the melts to reduce-aluminum and Mg oxidation. An advantage of this invention is that both physical and non-toxic physico-chemical mechanisms are utilized to stymie unwanted oxidation of the metal being processed.
Yet another object of the present invention is to provide a method for using both alloying and minor elements during aluminum secondary processing operations to grow well-defined oxides as barriers to further oxidation of aluminum products. An feature of this invention is that the imposition of a physical barrier between the melt surface and furnace atmosphere, combined with suitable alloying element concentrations, facilitates the growth of coherent, lattice-matched oxide films whose defect free nature reduces transport of melt components to the surface layer of the growing dross film. An advantage of the invention is the minimization of three-dimensional oxide growth and preventing the oxide from extending through the depth of the aluminum melt.
Briefly, the invention provides a method to minimize oxidation of metal during melting processes, the method comprising placing solid phase metal into a furnace environment; transforming the solid-phase metal into molten metal phase having a molten metal surface; and creating a barrier between the surface and the environment.
The invention also provides a method for isolating the surface of molten metal from its environment, the method comprising confining the molten metal to a controlled atmosphere; and imposing a floating substrate between the surface and the atmosphere.